ARGO at SINGAO: a multipurpose Astrophysical Radiation Observatory

PROCEEDINGS SUPPLEMENTS

Nuclear Physics B (Proc. Suppl.) 28B (1992) 15.5-162 North -Holland

A R G O at S I N G A O : a multipurpose Astrophysical Radiation Observatory O . C a t a l a n o a, J.Linsley a, and L.Scarsi a,b a istifuto di Fisica Gosmica ed Inyormatica dei C.N.R. Via M.Sfabile, 172 - 90139 P a l e m o , Ifaly b Dipartimen|o di Ene~etica ed Applieazioni di Fisiea - Unieersifd di Palermo

A presentation is given of an experimental facility designed for the study of the Primary Cosmic Radiation. The ARGO (Astrophysical Radiation and Gamma Observatory) detector consists of a continuous carpet (120x120 m2 ) of Resistive Plate Counters, capable of registering the arrival of individual particles in the EAS front with timing in the ns range and position accuracy in the order of cm's; the carpet is complemented by an underneath muon tracking multilayer of Limited Streamer Tubes of equal size. ARGO, located at mountain altitude, is conceived to operate with two main objectives: a) as a VHE-UHE Gamma ray telescope intercepting substantially the full Shower front and sized to extend upward (to >. 100 TeV) the upper energy limit of the air-Cerenkov telescopes set up at 10-20 TeV (Crab source), b) Operating as EAS front interceptor at distance from the shower core, ARGO is capable of providing relatively complete and detailed informations on both the electron and muon population in the EAS; the shower front coverage added to the function of muon tracking hodoscope, gives to ARGO the unique characteristics of a "WIDE ENERGY RANGE TELESCOPE (WERT)" [1] for the Primary Cosmic Rays, capable of "fertilizing" large areas ( increasing with shower size) and providing complete data on individual events from the rather well known region at the low energy r a n g e (1014 - l0 ts ) eV up to the presently high energy detectable limit at > 1019 eV. The project described is proposed by a Consortium of Italian Universities and Research Institutes (S.I.N.G.A.O.) for a location in Southerm Italy, at the Castelgrande Plateau [2].

is confuse and eontradictory~ - Cosmic R a y composition at E > 1014 - 10 ts

1. I N T R O D U C T I O N In reviewing the problematics of identification

eV: only fragile indications exists beyond the en-

a n d genesis of the P r i m a r y Cosmic Radiation,

ergy limit for direct observation of the primary

considered as m a d e up of its neutral ( G a m m a

particles: the situation becomes less and less def-

and Neutrino)

inite as the p r i m a r y energy increases.

and charged component

(con-

ventional nucleonic Cosmic Rays and electrons-

- Spectral behaviour of Cosmic Rays at E > - I 0 t 9 eV: detection and interpretation of

positrons), we see that, in spite of the consider-

I0 t8

able a t t e n t i o n which has been and is being given

possible directional anisotropies; apparently the

to this field, m a n y f u n d a m e n t a l problems remain

problems seems to derive from poor statistics,

open a n d some basic chapters are still in the age

but possibly this is only a part of the story. - Neutrino Astronomy: existence and origin of

of infancy. We list some of them: - V H E and U H E G a m m a R a y Astronomy: de-

a detectable flux: the only positive detections

tection o f c o m p a c t sources as well ofdifl'use emis-

have been t h a t of the low energy flux from the

sion a t E~ > 1 TeV. The only confi:rned source is

Sun and, possibly, the burst relate~ to the explo-

C r a b u p to 20 TeV, while the general p a n o r a m a

sion of SN 1987 A.

0920--5632/92/$05.00 © 1992- Elsevier Science Publishers B.V. All rights reserved.

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The main difficulties arise from the low flux values involved (the low interaction cross section in the case of neutrinos). Direct observation of primaries in space stops at ~. 100 GeV for Gamma astronomy and at ~ 1014 eV for the charged nucleonic component. Above these thresholds, the Earth atmosphere must be used as a converter-amplifier and the observations are ground based. Cerenkov light can be exploited to extend gamma observations up to 1012 - 1013 ev primary energies. Fluorescence intensity as a function of distance along the EAS trajectory yields the longitudinal structure of the shower, with the total light output giving the total energy dissipated in the atmosphere: with large installations (Fly's Eye) Cosmic Rays observations can be extended up to the highest energies. The limitation in both cases derives from the request of clear moonless nights, which reduces the ontime, and therefore the "effective area" to 10% of the maximum exposed; from that derive the upper limits on the observable energy spectra for Gamma sources and the conventional Cosmic Rays. To try to answer the open questions listed above by the detection of the E.A.S. front impinging at ground level, we need, in order to reach sufficient statistics, sensitive areas from > 104 m 2 for Gamma-ray astronomy, to several hundreds (up to thousands) Km 2 for the extreme end ( > 1020 eV ?) of the C.R. spectrum. Should the same detector be used for neutrino astronomy (detection of upward moving muons from neutrino interaction underground), taking into account the today expected source intensity, we would need Aefl" 104 - 105 m 2 at least. The instrumentation required and the coverage of the sensitive ares is naturally problem dependent. The general approach is however fundamentally the same, requiring the availability of a large ares with various degrees of exploits-

tion: exclusive occupacy of a central region (10 4 - 105 m2), with dedicated instrumentation scattered over or much larger surface. The experiment should be located at a reasonable height above sea level to favour the detection of the E.A.S. not far from the maximum longitudinal development. In any case a major effort is required, asking for important facilities and a strong and coordinated experimental program, beyond the possibility of isolated research groups. As a starting point of such a program the S.I.N.G.A.O. consortium is proposing to build a 1.Sx10 4 m 2 telescope (A.R.G.O.) optimized for the detection of primary gamma rays from point sources (Crab like) in the energy region (3100) TeV, which, by the complement of a muon tracking facility of equal size, can operate in a "Wide Energy" mode [1] to cover for CosmicRays E.A.S. (direction of arrival, longitudinal development, size) in the energy range 1014 - 1019 eV. In the shower front detection, a techno!ogicad change is made from the M.I.T. standard approach of particle density-arrival time sampling to identification, hit positioning and arrival timing for individual particles; the muon component is tracked and its arrival direction reconstructed to the point of origin in the shower development axis. ARGO makes extensive use of R.P.C. (Resistive Plate Counters) which provide particle hit position at cm scale and timing with ns accuracy, and of L.S.T. (Limited Streamer Tubes) giving the hit position at cm level; large scale readout and on-line processing electronics is used, together with advanced pattern recognition software. The intervention of industrial partners, qualified with experiments at CERN and Gran Sasso Laboratories, is foreseen .The Singao Consortium members are listed in Appendix I, together with the ARGO proposers. The proposed site for ARGO is Toppo di Castelgrande, 35 Km

0. Catalano et al. /ARGO at SINGAO

from the city of Potensa (Province of Basilicata), about midway between Naples and Bari, at 1200 m.a.s.l. [2].

2. ARGO experimental set-up A preliminary description of the ARGO telescope is given in a "Letter of intent", submitted in February 1992 to the Italian Istituto Nasionale di Fisica Nucleare by eight groups belonging to Italian Universities and Research Institutes [3]; we report here the relevant information. Fig. 1 shows the vertical cross-section of the main building which will house the central detector system together with the general facilities. The figures quoted are indicative and will be defined in the final constructions plans; the structural scheme of the building is based on 16 indipendent modules of 30x30 m 2 each. Fig.2 shows a view of the horizontal cross-section of the ARGO building; a possible surrounding array is also shown. Figures 1 and 2 give a schematic view of the proposed telescope. It consists of an upper single layer of Resistive Plate Counters (RPC) covering 120x120 m 2 a r e a and a minimum of two layers of Limited Streamer Tubes (LST) covering the same area under 2-3 m of rock. The upper RPC plane measures the space-time pattern of the particle population in the shower front; the good time resolution (2ns) of this detector allows rather precise timing at the level of single particles. The LST system placed below the rock absorber, acting as a muon-filter, allows to measure the muon content and provides accurate tracking for the particles, also by the addition of the RPC information. Fi8.2 shows the optiol, of surrounding the central telescope by an array of RPC-LST multilayered stations to recover edge effects and to increase the effective sensitive area.

157

~.I. The R P C sys¢em

RPC are thin gap devices operating in streamer mode in a very high, uniform electric field (40 KV/cm) [4,5]. A typical unit is made by two plates (lx2 m 2) of phenolic polymer faced up one another at a distance of 2 mm kept constant by PVC disk spacers and enclosed in a PVC frame to maintain gas thigthness. The external surfaces of the plates are coated with graphite to which high voltage and ground are applied through metallic contacts. The counter is filled with a mixture of argon, butane and freon through smell gas feed-througs on both sides of the PVC frame. The charge produced by the streamer process is picked-up by copper pads (12 cm x 12 cm) supported by a polycarbon structure facing the high voltage side. The large pulse height of the signal produced allows for very simple and inexpensive electronics. The operation Rat top is very long (~ 1000 V) and therefore a very high working stability is obtsined; single rates are at a level of few KHs/m 2 at 8.2 KV with a threshold of 60 mV and reach 10 KHs/m 2 at 9.0 KV. The mean cross-talk to side pads does not exceed 2%. The experimental results for time resolution give value around lns for a single pad. The whole floor of the experimental hall is covered by a mosaic of RPC chambers (lx2 m 2 each), for a total of 1.5 x 104 m2; the 128 pads/chamber are grouped in order to obtain the digital pattern with a spatial resolution of 12x12 cm 2 and the time information of the first particle hitting each I m 2 a r e a . To convert the photon component, it is planned to cover the RPC floor with s IXo of Fe.

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Tubes SysZem

Two (as a minimum), with an option up to four, complete layers of streamer tubes will be located in the lower floors of the building spaced at a distance of the order of meters. The tubes 3x3x600 cm 3 are identical (apart from the lenght of 6 m instead of 12 m) to those used in MACRO [6] and EAS-TOP [7] experiments at Gran Sasso. They are equipped with pick-up strips 3 cm wide, consenting the (x,y) location of the particle hits with a 3x3 cm 2 resolution. It is important to stress thai, this streamer system will result in the by far the largest continuous coverage for muon detection existing in the world, providing moreover the_ accurate tracking of the particle trough the correlation, with pattern recognition systems, of the hit positions in each layer; the ex-

tension of the track, to the RPC layer, will provide, with ns accuracy the arrival time for each particle.

~.8. The su~p'ounddng a~r'up The 120x120 m 2 ARGO central detector will be surrounded by an E.A.S. sampling array; this array is not yet defined and it can be conceived along different lines. Fig.2 shows a possible example: 120 sampling stations, each made by 8 RPC'S (2xl m 2 each) arranged in a 4x4 m 2 system and it is designed to recover the edge elfects for showers with the axis at the periphery of the central building, with a substantial increase of the effective area (almost a factor of 2), by adding a section with 15% sampling fraction. This configuration can complement ARGO

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when operating as Gamma-ray telescope. Other arrangements can be obtained with wider spacing and more complete stations (e.g. of the Plasrex type, [8,9] with s mu!ti!~ver RPC-LST dispo. sition and an intermediate Pb/Fe converter, providing timing and tracking l'or individual shower particles). We remark that the Castelgrande Iocation allows area occupacy up to 10 Km 2 with complete control and up to thousands Km 2 with no severe limiting conditions [2] The upper and lower experimental halls offer large areas and volumes, which can be filled with additional kind of instrumentation (TRD, Scintillation counters,...) for a better knowledge of shower features.

3. ARGO as a VHE-UHE Gamma-ray telescope This aspect is beyond the scope of this workshop; we limit therefore ourselves to report the b u i c informations. The main parameters characterizing the response of the telescope to incident showers: trigger efficiency vs primary energy, angular resolution, gamma vs hsdron discrimination through the measurement of the muon content, have been studied by means of Monte Carlo simulations [3]. Calculations have been performed l'or an elevation of 1200 m.a.s.l, and a zenith angle of 30 degrees.

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O. Catalano et aL l A R G O at SINGAO

(TeV) 2.5 5 10 2O 5O

A 0 (degrees) 2.6x10 3

0.6

5.2x1603 1.1xl004 2.3x1004 3.2x1004

0.5 0.2 0.2 0.2

A discriminatio;~ fe~tor gamma/hadrons better than 10-4 is expected at energies greather than 50 TeV, pushing the sensitivity of ARGO very close to the expected flux of the diguse isotropic ~amma-rays. Fig.3 shows the observable gamma ray flux at 5 standard deviations level in one year of data taking, whithout muon rejection ( - - - ) and with muon rejection (-). The measured flux f~om the Crab (Wipple: W; Themistocles: Th and upper limits at higher energies: Tibet-T; Los Alamos-C; HEGRA-H; Fly's Eye-F; EAS-TOP-E, UMC-U) are shown for comparison.

4. ARGO as a "Wide Energy Range Telescope (WERT)" The Extensive Air Showers offer the macroscopic aspect of the single energetic particles present in the Primary Cosmic Radiation flux impinging on the top of the atmosphere; because of large cross section of the phenomenon, EAS's ~Uow the extension of the upper limit of the obser~able Cosmic Ray e~?rgy spectrum upward from the 10 t4 - 10 tS eV value observable by directly intercepting the particles with space born detectors. While the "direct" observation is in principle, able to identify the characteristic parameters: energy, A,Z and direction of arrival, toil has to be payed when the observation is indirect, through the secondary E.A.S., with a loss in "resolution", expecially in respect to the determination of A and Z. The measurement of

the "primary" parameters is dependent on the knowledge of the longitudinal development of the cucade and the particle composition, lateral distribution, arrived direction and time structure for the shower front when intercepted by ground based detectors. A general overview on this problematics is given by M.HUlas in his contribution to this workshop [10]. We draw attention to the capacity of ARGO in sampling with a statistically significant weight, because of its large sensitive area, the shower front at distances up to several km's from the axis for both the population space distribution and structure of arrival time; for the ~mon component accurate tracking and timing is provided. Extensive experimentation with tracking detectors operating insid~ the GREX scintillator array at Haverah Park [8][9][11] have demonstrated the possibility of tracing back the muon (and high energy electron) trajectories to find the height at which the originating interaction occurred. By using its timing-tracking capability, for the muon component, added to that of particle timing and positioning for the entire population on the shower front section intercepted, Argo can operate as an "all-time" counterpart to the Fly's Eye operation in reconstructing the longitudinal EAS development. A more detailed description of ARGO as a Wert is given by Catalano et 81 [1 ] in these Proceedings. Although same caution has been put forward by Hillas [10] about the sensitivity of the "heights of origin" (or ejection from the shower axis) in giving a good overall picture of the c~cade, we feel confident that, by adding to the "tracking" also the ir,di.-'idual timing for the rouen component and the detailed knowledge of the accompanying electron-photon component, ARGO-WERT will act as an efficient "resolution" oriented telescope in condition to answer many of the "primary composition" open questions still remaining between 1015 eV

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Fig, 3. Crab gamma ray flux and > 1019 ev [12]. In general it appears that ARGO could set as the necessary complement to the "thousand" Km 2 array, with conventional density-timing sampling proposed in this workshop, the first empitasising the "composition" aspect with a limit in the effective-areas and therefore statistics at the high energy limit, the second "statistics" oriented at 1020 ev, but "resolution" limited.

low for the mass production (~ 6000 chambers) required by Argo; this is being done through the intervention of qualified industrial firms opersting in the field of nuclear physics instrumentstion. The "electronics" is being investigated and prototyped in the research Institutes involved in Argo, in strict connection with potential industrial partners. For the LST, the units are comm~ercially available. In genera], the quality/cost aspect is considered the main driver.

5. Status of the project

5.~, Funding and approval

5.1. Technical

A first approximation cost estimate gives 35 Billions Life for the Buildings and infrastructures (to be provided under the specisi in~er~'e~tion Law for the development of Southern Italy)

The RPC technology is now undergoing the process of engineering and standardisstion to al-

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O. Catalano et al. / A R G O m SlNGAO

and 30 Billions Life for the Experiment. A ]etter of intent for the "Experiment" has been submitted to INFN (Istituto Nazionale di Fisica Nudeare - for Italy) on February 1992 for approval and inclusion in the 5 year plan 1993-1097; a detailed proposal will be submitted at end 1992. 5.3. L o c e t i o n

Fisica • Sezione INFN, Universit~ di Roma II "Tot Vergata" M.Guida, F.Mancini Dipartimento diFisic Teorica, Universit~ di Salerno P.Ghia, G.P.Mannocchi, O.Saavedra, P.'I~ivero Istituto di Fisica Generale, Universit/L di Torino Istituto Cosmogeofisica CNR, Torino

A detailed description of the location is given

in [2]. APPENDIX I ARGO at SINGAO A gamma-ray telescope operating in the 3-100 TeV energy range M.Abbrescia, G.Iasdli, S.Natali, S.Nusso, A.Ranied, F.Romano Dipartimento di Fisics e Sezione INFN, Universit~ di Bari G.Auriemma, C.Satriano Universit~ degii Studi della Builicata, Potensa P.Bemardini, P.Creti, G.Mancarella, S.Petrera, P.Pistilli, A.Surdo Dipartimento di Fisica • Sesione INFN, Universit4 di Lecce M.Ambrosio, G.C.Barbarini, B.Bartoli, D.Campana, B.D'Ettorre-Piusoli, G.Di Sciascio, F.Guarino, M.Iacovacci, G.Osteria, V.Silvestdni Dip~timento di Fisica • $esione INFN, Universit~ di Nspoli "Federico II" G.Agnetta, R.Burlon, O.Catalano, G.D'AII, J.Linsley, G.Richiusa, L.Scarsi Universit~ di Palermo e IFCAI-CNR G.Bressi, A.Lanza Dipartimento di Fisica • Sezione INFN, Universit~ di Pavia M.De Vincensi, E.Lamanna, P.Lipari, M.Severi Dipartimento di Fisica e Sezione INFN, Universit~ di Roma I "La Sapiensa" R.Cardarelli, R.Santonico Dipartimento di

References [1] O.Catalano, J.Linsley and L.Scarsi: These Proceedings. [2] G.Auriemma,F.Lorusso and C.Satriano: These Proceedings. [3] M.Abbrescia et al. Letter of intent for "ARGO at SINGAO", Submitted to INFN - Feb. 1992 [4] R.Santonico et al., Nucl.Inst.and Mefh. IST, 377 (19gl). [5] R. Cardarelli et al., Nuci.insf.andMeth. A.268, 20 (1988). [6] MacroCollaboration. Proc.21st ICRC (Adelaide) I0 - 316 (1990) [7] M.D'Aglietta et al. Proc.2Oth ICRC (Moscow) 6 510 ( 1 9 8 ~ ) [8] G.Agnetta et al., If Nuo~o Cime~|o 18C, 391 (1990). [9] G.Agnettaet al. Proc. 22rid ICRC (Dublin) OG 10.3 11 [I0] A.M.HilIas.These Proceedings [11] G.Agnetta et al. To be published [12] J.Linsley. These Proceedings -